This invention relates generally to fuses, and more particularly to chip fuses for protecting electronic devices from damaging electrical currents.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Typically, fuse terminals or contacts form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals or contacts, so that when electrical current through the fuse exceeds a predetermined threshold, the fusible elements melt, disintegrate, sever, or otherwise open the circuit associated with the fuse to prevent electrical component damage.
A proliferation of electronic devices in recent times has resulted in increased demands on fusing technology. For example, a conventional fuse for electronic application includes a wire fuse element (or alternatively a stamped and/or shaped metal fuse element) encased in a glass cylinder or tube and suspended in air within the tube. The fuse element extends between conductive end caps attached to the tube for connection to an electrical circuit. However, when used with printed circuit boards in electronic applications, the fuses typically must be quite small, and tend to require leads which may be soldered to a circuit board having through-holes therein for receiving the leads. Miniature electronic fuses of this type are known and can be effective in protecting electronic circuitry.
At least in part to avoid manufacturing and installation difficulties of miniature electronic fuses, chip fuses have been developed which may be surface mounted to circuit boards, thereby eliminating the fuse tube and the lead assemblies, while at the same time providing better fusing characteristics (e.g., faster acting fuses) for some electronic circuits. Such chip fuses may include, for example, a substrate layer, a fuse element layer, one or more insulative or protective layers overlying the fuse element layer, and end terminations formed over the substrate and the fuse element layer for surface mounting to a circuit board. While known chip fuses provide low cost fuse products that are easily soldered to circuit boards, they are difficult to replace when a fuse opens to interrupt an electrical circuit, and as the boards are generally inoperable due to the opened fuse, the circuit boards, and sometimes the entire electronic device associated with the circuit boards, are typically discarded. This is problematic, however, in certain installations involving expensive boards and equipment for which disposal is not a practical option.
For example, known memory chips for computer and processor applications typically must be tested prior to use, and one of the tests is known as a burn-in test. The burn-in test evaluates continuous operation of the memory chip before being put to use, and the memory chips are typically subjected to an elevated temperature above anticipated operating electrical conditions. Stability issues, defects, and early-life failures of the memory chips may be revealed in the burn-in test.
In one known burn-in testing system, a large number of memory chips are connected to a large circuit board with chip sockets that receive the memory chips, and chip fuses are used to protect the electronics of the system in the event of failure of the memory chips. When one or more of the chip fuses opens, the associated chip socket or sockets becomes inoperable, and because the chip fuse is surface mounted to the board, it is not practical to remove and replace the operated fuse or fuses. To allow the chip sockets to be used nonetheless, it has been proposed to provide though-holes in and/or fuse sockets or clips mounted to the board of at least one known testing system so that miniature electronic fuses may be mounted to the board with leads of the fuses being received in the through-holes and secured with, for example, the fuse sockets or fuse clips to establish an electrical connection in parallel with the operated fuse. While the miniature electronic fuses may be a viable solution to restoring use of a chip socket or sockets affected by an operated chip fuse, known miniature electronic fuses having leads are disadvantaged as they tend to have different operating characteristics than chip fuses, and therefore the two types of fuses perform differently and provide non-uniform protection of the electrical system.